1. Field of the Invention
The present invention relates to a blood pressure estimating apparatus for successively estimating an intraarterial blood pressure of a living subject, based on information non-invasively obtained from the circulatory organ of the subject.
2. Related Art Statement
There is known, as a blood pressure (BP) measuring apparatus for non-invasively measuring an intraarterial blood pressure of a living subject, a so-called Korotokoff-sound-type BP measuring apparatus or oscillometric-type BP measuring apparatus. The Korotokoff-sound-type BP measuring apparatus automatically determines a blood pressure of the subject, based on a pressing pressure of a pressing band being wound around a body portion of the subject at the time of change of Korotokoff sounds produced by changing the pressing pressure of the pressing band. The oscillometric-type BP measuring apparatus automatically determines a blood pressure of the subject, based on the variation of amplitude of pulse wave produced by changing the pressing pressure of the pressing band.
In an operating room, an intensive care unit, or the like, it may be needed to measure successively a blood pressure of a patient because an urgent medical treatment or cure may be needed. In the case, however, where the conventional BP measuring apparatus is used, it takes several tens of seconds from the start of blood pressure measurement to obtain a blood pressure of the patient. If an interval between successive blood pressure measurements is shortened to obtain a blood pressure at a short period, congestion may occur to the body portion of the patient due to the increased frequency of pressing of the pressing band, whereby errors may occur to the measured blood pressure values.
Meanwhile, there has been proposed a BP estimating apparatus including means for determining a velocity of propagation of a pulse wave which propagates through an artery of a living subject, and means for successively estimating, according to a predetermined relationship between blood pressure and velocity of propagation of pulse wave, an intraarterial blood pressure of the subject, based on the determined velocity of propagation of the pulse wave. An example of the BP estimating apparatus is disclosed in Laid-open Publication No. 7-9305 of unexamined Japanese Utility Model Application and Laid-open Publication No. 7-308295 of unexamined Japanese Patent Application.
However, the above BP estimating apparatus has only the function of estimating successively a blood pressure based on a propagation time of a pulse wave or a propagation velocity of a pulse wave. In the case where the blood pressure is estimated based on only the pulse-wave propagation time or the pulse-wave propagation velocity, the estimated blood pressure can not enjoy high accuracy. Therefore, it is needed to frequently calibrate the BP estimating apparatus, based on an actual blood pressure measured by the Korotokoff-sound-type BP measuring apparatus or the oscillometric-type BP measuring apparatus.
It is therefore an object of the present invention to provide a blood pressure estimating apparatus which estimates, with high accuracy, a blood pressure of a living subject.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided an apparatus for successively estimating an intraarterial blood pressure of a living subject, based on information non-invasively obtained from a circulatory organ of the subject, comprising a blood-pressure measuring device which includes an inflatable cuff adapted to apply a pressing force to a body portion of the subject and iteratively measures, by changing the pressing force of the cuff, a blood pressure of the subject at a predetermined period; a first means for non-invasively obtaining, from the circulatory organ of the subject, a set of first information which relates to a velocity of propagation of a pulse wave which propagates through an artery of the subject, at least each time the blood-pressure measuring device measures a blood pressure of the subject; a second means for non-invasively obtaining, from the circulatory organ of the subject, at least one of a set of second information which relates to a heart rate of the subject and a set of third information which relates to an area defined by a volume pulse wave from a peripheral portion of the subject, at least each time the blood-pressure measuring device measures the blood pressure; a first memory device which stores a plurality of groups of information each group of which comprises the blood pressure measured by the blood-pressure measuring device, the set of first information obtained by the first means when the blood pressure is measured by the blood-pressure measuring device, and the at least one of the set of second information and the set of third information obtained by the second means when the blood pressure is measured by the blood-pressure measuring device; an expression determining means for determining an expression representing a relationship between (A) blood pressure, and (B1) first information and (B2) at least one of (B21) second information and (B22) third information, the expression having a first coefficient for the first information, at least one second coefficient for the at least one of the second information and the third information, and a constant, the expression determining means determining the first and second coefficients and the constant of the expression by applying a multiple regression analysis to more than a first predetermined number of the groups of information last stored in the first memory device, the first predetermined number being equal to a total number of the first coefficient, the at least one second coefficient, and the -constant; and a blood-pressure estimating means for successively estimating, according to the determined expression representing the relationship between (A) blood pressure, and (B1) first information and (B2) at least one of (B21) second information and (B22) third information, an intraarterial blood pressure of the subject, based on each of a plurality of sets of first information successively obtained by the first means and at least one of each of a plurality of sets of second information, and each of a plurality of sets of third information, successively obtained by the second means when the each of sets of first information is obtained by the first means.
In the present BP estimating apparatus, the BP estimating means successively estimates, according to the expression representing the relationship between (A) blood pressure, and (B1) first information and (B2) at least one of (B21) second information and (B22) third information, an intraarterial blood pressure of the subject, based on each set of first information successively obtained by the first means and each set of second and/or third information successively obtained by the second means. Therefore, as compared with the case where a blood pressure is estimated based on only the first information, the present apparatus estimates the intraarterial blood pressure, based on, in addition to the first information, at least one of the second information as a parameter on the side of the heart of the subject that changes in relation with the blood pressure and the third information as a parameter on the side of peripheral portion of the subject that changes in relation with the blood pressure. Accordingly, the accuracy of the estimated blood pressure is improved.
In addition, the first memory device stores, at the predetermined period, a group of information comprising a blood pressure measured by the blood-pressure measuring device, a set of first information obtained by the first means when the blood pressure is measured by the blood-pressure measuring device, and at least one of a set of second information and a set of third information obtained by the second means when the blood pressure is measured by the blood-pressure measuring device, and the expression determining means determines the first and second coefficients and the constant of the expression by applying a multiple regression analysis to more than a first predetermined number of groups of information last stored in the first memory device, the first predetermined number being equal to a total number of the first and second coefficients and the constant. The first predetermined number is counted retrospectively from the last group of information that are last stored in the first memory device. Thus, any dispersion included in the groups of information stored in the first memory device can be well removed, and the expression or the relationship can be so determined as to be suitable for the individual subject. Here, it is noted that the first information and the second and/or third information obtained when the blood pressure is measured by the BP measuring device may change because of the change of physical condition of the subject, such as respiration or arrhythmia, or the occurrence of artifact noise. That is, the groups of information stored in the first memory device may include the dispersion which does not result from the change of blood pressure of the subject. In contrast, if the expression or the relationship is determined based on only the first predetermined number of groups of information stored in the first memory device, the determined expression or relationship is influenced by the dispersion of the groups of information that does not result from the change of blood pressure of the subject.
According to a second feature of the present invention, the expression determining means comprises means for determining the first and second coefficients and the constant of the expression by applying the multiple regression analysis to not more than a second predetermined number of the groups of information last stored in the first memory device, the second predetermined number being greater than the first predetermined number. The first predetermined number may be three or four, and the second predetermined number may be from 10 to 20, preferably 20.
The second predetermined number corresponds to a time duration (e.g., 60 minutes) in which, it can be expected, a relationship between the BP values measured by the BP measuring device and the sets of first information obtained by the first means does not change basically. That is, in a short duration, the BP values and the sets of first information keeps a good correlation; but, in a long duration, this correlation gradually decreases because of the change of physical condition of the subject (e.g., a patient), such as the condition of circulatory organ. Therefore, if the expression or the relationship is determined based on the groups of information which include one or more groups of information which were stored in the first memory device, an excessively long time before, the determined expression or relationship may not be suitable for the current physical condition of the subject.
According to a third feature of the present invention, the second means obtains the set of second information and the set of third information, and the blood-pressure estimating means comprises means for estimating, according to the expression representing the relationship between (A) blood pressure, and (B1) first information, (B21) second information, and (B22) third information, the intraarterial blood pressure of the subject, based on the each set of first information obtained by the first means, and each set of second information and each set of third information obtained by the second means when the each set of first information is obtained by the first means. In this case, it is not needed to frequently calibrate the estimating apparatus, based on an actual blood pressure of the subject measured by using the cuff, because the estimated blood pressure enjoys higher accuracy in comparison with an estimated blood pressure which is estimated based on only the first information.
According to a fourth feature of the present invention, the first means comprises means for obtaining, as the set of first information, a time, DT, which is needed for the pulse wave to propagate between two different portions of the artery, the second means comprises means for obtaining, as the set of second information, a heart-beat period, RR, of the subject, and means for obtaining, as the set of third information, a ratio, VR, of the area of the volume pulse wave to the heart-beat period RR, and the blood-pressure estimating means comprises means for successively estimating, according to the expression representing the relationship between (A) blood pressure, EBP, and (B1) time DT, (B21) period RR, and (B22) ratio VR, defined as follows: EBP=xcex1(1/DT)+xcex2RR+xcex3VR+xcex4, where xcex1 is the first coefficient, xcex2 and xcex3 are the second coefficients, and xcex4 is the constant, an intraarterial blood pressure of the subject, based on each of a plurality of values of the time DT successively obtained by the first means, and each of a plurality of values of the period RR, and each of a plurality of values of the ratio VR, successively obtained by the second means when the each value of the time DT is obtained by the first means. The two different portions of the artery may comprise the heart and the capillaries of the subject. In the present apparatus, the BP estimating means estimates the intraarterial blood pressure based on the second information as the parameter on the side of the heart of the subject and the third information as the parameter on the side of the peripheral portion of the subject as well as the first information. In this case, it is not needed to frequently calibrate the present apparatus, based on an actual blood pressure of the subject measured by using the cuff, because the estimated blood pressure enjoys higher accuracy in comparison with an estimated blood pressure which is estimated based on only the first information.
According to a fifth feature of the present invention, the estimating apparatus further comprises a second memory device which stores at least one group of predetermined first and second coefficients xcex1, xcex2, xcex3 for the expression that are predetermined by applying the multiple regression analysis to a plurality of groups of information which are obtained from a plurality of persons and each group of which comprises a blood pressure obtained from a corresponding one of the persons, and a time DT, a period RR, and a ratio VR which are obtained from the one person when the blood pressure is obtained from the one person, and, before the first memory device stores more than the first predetermined number of groups of information, the blood pressure estimating means successively estimates, according to the expression having the one group of predetermined first and second coefficients xcex1, xcex2, xcex3, an intraarterial blood pressure of the subject, based on each of a plurality of values of the time DT successively obtained by the first means and each of a plurality of values of the period RR, and each of a plurality of values of the ratio VR, successively obtained by the second means when the each value of the time DT is obtained by the first means. In this case, the present estimating apparatus can obtain, even in an early period of operation thereof, a widely applicable expression or relationship with which the apparatus can successively estimate a blood pressure of the subject with accuracy.